The present invention relates to an antenna arrangement comprising a substantially planar patch conductor, and to a radio communications apparatus incorporating such an arrangement.
Wireless terminals, such as mobile phone handsets, typically incorporate either an external antenna, such as a normal mode helix or meander line antenna, or an internal antenna, such as a Planar Inverted-F Antenna (PIFA) or similar.
Such antennas are small (relative to a wavelength) and therefore, owing to the fundamental limits of small antennas, narrowband. However, cellular radio communication systems typically have a fractional bandwidth of 10% or more. To achieve such a bandwidth from a PIFA for example requires a considerable volume, there being a direct relationship between the bandwidth of a patch antenna and its volume, but such a volume is not readily available with the current trends towards small handsets. Further, PIFAs become reactive at resonance as the patch height is increased, which is necessary to improve bandwidth.
A further problem occurs when a dual band antenna is required. In this case two resonators are required within the same structure, which means that only part of the available antenna area is used effectively at each frequency. Since the bandwidth of an antenna is related to its size, even more volume is required to provide wideband operation in two bands. An example of such an antenna is disclosed in European patent application EP 0,997,974, in which two PIFA antennas are fed from a common point and share a common shorting pin. The low frequency element is wrapped around the high frequency element, which therefore means that the high frequency element must be small compared to the total antenna size (and therefore narrow band).
Our co-pending unpublished United Kingdom patent application 0101667.4 (Applicant""s reference PHGB 010009) discloses a variation on a conventional PIFA in which a slot is introduced in the PIFA between the feed pin and shorting pin. Such an arrangement provided an antenna having substantially improved impedance characteristics while requiring a smaller volume than a conventional PIFA.
An object of the present invention is to provide an improved planar antenna arrangement.
According to a first aspect of the present invention there is provided an antenna arrangement comprising a substantially planar patch conductor, having first and second connection points for connection to radio circuitry and a slot incorporated between the points, and a ground plane, wherein the antenna arrangement operates in a plurality of modes depending on the impedances of the circuitry connected to the first and second connection points.
By varying the impedances connected to the connection points, the current fed into the antenna may follow different routes, thereby providing different modes of operation. The modes may have different resonant frequencies and/or different impedances. The impedances may include short and open circuits, which may be provided by switches or passive circuits. Further connection points may be provided, and the radio circuitry may comprise a distributed diplexer. All of these arrangements have the advantage of enabling a reduced antenna volume compared to a PIFA of equivalent volume by making full use of the patch conductor in all modes.
According to a second aspect of the present invention there is provided a radio communications apparatus including an antenna arrangement made in accordance with the present invention.
The present invention is based upon the recognition, not present in the prior art, that by enabling the impedances connected to points on the patch conductor of a PIFA to be varied, dual-band and multi-band antennas making full use of the patch area in all bands are enabled.